The present invention relates to conveyors of the type wheren a plurality of serially arranged loaded pallets are supported during storage on and are moved by gravity down a rollerway conveyor comprising parallel flights of a plurality of rollers having parallel, coplanar and horizontal axes, and with the flights arranged in a generally common plane inclined in the direction of conveying. Such a rollerway gravity conveyor may be of the type as shown in the patent to Andrew T. Kornylak issued May 13, 1969 as U.S. Pat. No. 3,443,674.
The conveyor of the above-mentioned patent has a plurality of rollers with annular elastomeric tires mounted on hubs that are each mounted by roller bearings on an axle. The anti-friction bearing is used to reduce the rolling resistance of the roller and to try and keep the rolling resistance uniform over a range of different loads. The compressable tire is desirable to permit defects or other height variations in the pallet bottom surface to ride "through" the tire instead of having to climb over it. This results in a reduced slope requirement for gravity rollerways and a smoother pallet movement for all types of roller conveyors. It is desirable to maintain the slope of the gravity rollerway conveyor as small as possible while accomplishing the purposes of the conveyor, because as the slope is increased, there will be a corresponding increase in the acceleration of a loaded pallet freely moving down the rollerway; if no or a few pallets are on the rollerway and a loaded pallet is inserted at the upper end of the rollerway conveyor, a relatively large slope for the conveyor will mean that the loaded pallet will gain considerable speed as it moves down the rollerway conveyor to present a safety problem as well as a problem in trying to stop the loaded pallet at the bottom of the coveyor without damaging either the conveyor or the load.
Conveyor rollers with an annular resilient or elastomeric tire may be constructed so as to have a substantial amount of hysteresis, which is the ability of the tire to absorb energy from the load to control the speed of the movement of a load down a gravity conveyor, particularly, to absorb more energy with higher speed to either slow down a fast moving load or prevent a load from building up speed as it moves down a gravity rollerway conveyor. An example of such high hysteresis is found in the United States patent to Andrew T. Kornylak issued Nov. 23, 1971 as U.S. Pat. No. 3,621,960. Such hysteresis rollers provide effective speed control for a specific range of loads. However, it has been found that in cold temperature environments (cold temperature is defined herein as temperatures below freezing), the elastomeric materials have very substantial compression set properties. Compression set is measured according to ASTM Test Method B. Elastomerical resilient materials for constructing the tires of the rollers of a gravity rollerway conveyor are most successful when they employ the lowest compression set while at the same time having the remaining desirable characteristics of such tires, and it is desirable that compression set be 5% or less (ASTM Test Method B), which is defined herein as no appreciable compression set. However, tires having a compression set of 5% or less at room temperature take on a vary substantial compression set in cold temperature storage and many times when the load is removed, a flat spot remains on the tire regardless of the amount of time allowed for recovery of the tire. Also, the recovery may be sufficiently slow such that movement of the pallets one space in the storage serial array may be finished before the tire has recovered sufficiently from its compression set, so that the flat spot will be present during movement of the pallets. Futher, with compression set of the tires during storage, it is extremely difficult to start up the remaining pallets upon removal of the lowermost pallet, and for this reason it may be necessary to increase the slop of the conveyor beyond that which is otherwise desirable.
The compressability of the tire, which may be measured as its percent of compression for a given load has nothing to do with its compression set, which compression set as related to the amount of time needed for recovery of the tire to its original shape after the load is removed. A further characteristic of an elastomeric tire is its hardness, and it has been found desirable that elastomeric tires have a range of hardness of 65A to 90A Shore durometer. Tire hardness is not necessarily related to compression set. The creep characteristics of a tire used on a rollerway conveyor are such that as a load remains on a tire, the deflection of the tire gradually increases. When the load is removed, the tire shows a flat spot, which does not return to the original cylindrical shape within a reasonable time. This flat spot (compression set) resists rolling of the load and is therefore detrimental. Compression set may be overcome with rigid rollers, but rigid rollers require that an uneven spot of the pallet must climb up over the roller and as a result may become hung up on such a roller and prevent conveying. Rolling resistance of a conveyor roller for a gravity rollerway conveyor is defined as the pull required to move a load supported by such a roller, as measured at substantially zero speed to eliminate the effect of hysteresis. Thus, hysteresis will not affect the rolling resistance value of tire materials, which rolling resistance value varies greatly for different elastomeric materials used for roller tires. While hysteresis is a desirable property for speed control, rolling resistance is almost always an undesirable property, but a necessary evil of having a resilient tire. Rolling resistance will increase, in general, for an increase in wheel load, which is undesirable for a gravity conveyor. Also, rolling resistance to the start up of a stored load will increase substantially with increased compression set.
If the inclination of a particular gravity conveyor is set up so that a load under restraint will reliably start moving when the restraint is removed (that is the inclination is sufficiently great) while at the same time the load will not gain excess speed over the length of its movement (that is, the inclination is sufficiently small) with the satisfactory compromise of these conflicting characteristics, the conveyor may be totally unsuited for a load of substantially different weight per wheel. Use in low temperature environments greatly complicates this problem.